Palaeochannel Evolution, Northwestern Gawler Craton, South Australia B

PALAEOCHANNEL EVOLUTION, NORTHWESTERN GAWLER CRATON, SOUTH AUSTRALIA B. Hou1, L.A. Frakes2 and N.F. Alley1 1. Primary Industries and Resources South Australia, PO Box 1671, Adelaide, SA 5001 2. University of Adelaide, Adelaide, SA 5005 [email protected] INTRODUCTION (Figure 1) have been mapped at the 1:250 000 and 1:1 000 000 Tertiary palaeochannels, mostly discharging into the Eucla Basin, scale (e.g. Benbow, 1993; Rogers, 2000). Tertiary sediments are widely distributed in the northwestern Gawler Craton (Benbow in the northwestern Gawler Craton and adjoining Eucla Basin et al., 1995). The palaeochannels present numerous exploration margin infill the sites of palaeorivers and depressions carved into problems and opportunities, challenging geological, geophysical much older sediments and basement of the interfluves. and geochemical exploration techniques. The channel fills Geomorphology have experienced varying degrees of weathering and styles of Most of the area is covered by Quaternary sand plains and dunes sedimentation. The following summary is based on palaeochannel and regolith sheets, which obscure the Tertiary palaeochannels studies in the region, including palaeochannel–landform mapping, (Benbow, 1990). One of the most significant geomorphic features palaeochannel characterisation and sedimentological studies in the area is a string of playa lakes and lunettes developed (Clarke and Hou, 2000; Hou et al, 2000, 2001a,b,c). in topographic lows, below which generally lie the Tertiary palaeodrainage systems. Other prominent landforms in the REGIONAL SETTINGS area include the parallel Ooldea and Barton ranges and the Geology interconnecting Paling Range (Figure 1), which are well preserved The northwestern Gawler Craton is mainly composed of coastal dunes or barriers of Late Eocene or Early Oligocene Precambrian gneiss, schist, granite and banded iron-formation, age (Benbow, 1990; Clarke and Hou, 2000). These remarkable locally overlain by Palaeozoic, Mesozoic and Tertiary sedimentary landforms can be interpreted by considering their topographic rocks (Benbow, 1993). Tertiary palaeodrainages in the region relief, by applying Digital Elevation Models, and by interpreting

Figure 1. Palaeogeography of the palaeochannels draining the Gawler Craton region, showing the distribution of Tertiary marine, marginal marine, and channel sediments. Palaeochannel positions have been outlined by topography, geological maps, basement contours, drillhole data, multispectral scanner imagery, DEM, TEM, gravity, and existing palaeodrainage maps. (after Hou et al., 2000)

© CRC LEME 2003 Gawler Craton palaeochannels Page 1 Landsat and NOAA imagery (Hou et al., 2000). Other major CHANNEL FILLS geomorphic features, such as ancient headlands and embayments, Tertiary sedimentation was widespread in the area, leading to the are also recognisable in the Tertiary coastal area. deposition of four major units (Hou et al., 2003): the Eocene Pidinga Formation (mainly fluvial carbonaceous sediment), Khasta Weathering and erosion Formation (estuarine sands) and Ooldea Sand (coastal barrier The depth of weathering is highly variable, ranging from a few sands), and the Miocene Garford Formation (dominantly lacustrine metres to more than one hundred metres in bedrock-dominated clays). Although the lateral and vertical variation in lithology areas (Benbow et al., 1995). Lintern and Sheard (1998) described of the channel-fills makes intraformational correlation difficult, many examples of weathering profiles in the Gawler Craton, and these four major units can readily be recognised and correlated provided an outline of regolith terminology. Deep weathering throughout the region (Hou et al., 2001a). The thickness of profiles capped by siliceous and ferruginous duricrust (Benbow channel fills normally ranges from several metres to 50 metres, et al., 1995) have developed over different lithologies, including but locally exceeds 80 metres. The sedimentary facies change the crystalline basement and pre-Tertiary cover. Observations markedly both vertically and laterally within the channels due of weathered Quaternary cover sediments, unweathered Tertiary to the complexity of fluvial sedimentation (Figure 2). Facies channel fills, and deeply weathered basement rocks in drillholes relationships are complex between the principal units of the in the palaeochannels imply a major episode, or possibly several palaeochannels. Boundaries between units are often erosional episodes, of deep weathering and erosion (Alley et al., 1999). and in places a channel contains only one of these diverse rock Siliceous and ferruginous rocks include both groundwater and types, testifying to the intermittent character of stream activity pedogenic forms. Lateritic duricrust forms a blanket over most of (Figure 2). the area; however, the thickness and composition of the lateritic duricrust vary with topographic position. PALAEOCHANNEL EVOLUTION Physiography The palaeorivers drained a large catchment in which there were The area lies on the southwestern slopes of the Gawler Craton a wide variety of sedimentary environments (Figure 1). The drainage divide, in the arid zone of South Australia. The area has Pidinga Formation was deposited in a range of environments, from an annual rainfall of about 150 mm, with cold, wet winters and fluvial, estuarine to marginal marine. Carbonaceous materials hot, dry summers. The Stuart Range forms a regional drainage attest to a dense vegetation cover supported by a humid, tropical divide between the Eyre Basin to the east and the Eucla Basin to subtropical climate (Alley et al., 1999). The Khasta Formation to the southwest. A number of elongate depressions extending was deposited in estuarine to marginal marine environments. between the Stuart Range and the Eucla Basin delineate a series Locally, the Ooldea Sand in the Ooldea, Paling and Barton of Tertiary–Quaternary drainage systems (Figure 1). These ranges were deposited in energetic upper shoreface and beach depressions are now largely filled with aeolian sand sheets and environments during development of the Tertiary coastal barriers. dunes. Scattered throughout the length of these topographic The interdune area was probably occupied for some time by a lows are numerous playas and salinas commonly underlain with large lagoon (Immarna lagoon in Figure 1). The Garford clay and silcrete, calcrete or gypcrete. carbonate sequences were deposited in extensive lakes and rivers. Reversal of flow is inferred from the Garford Formation; from PALAEOCHANNELS westerly isolated and abandoned channel and lacustrine deposits, A number of palaeochannels have long been recognised and to easterly relatively continuous and elongate channel deposits. regarded as parts of larger, ancient drainage systems which flowed Subsequent drying of these lakes and channels led to deposition west or southwestwards to the Eucla Basin in Eocene to Miocene of regressive dolomitic clastic and chemical sediments of the times (Figure 1). The dimensions of the channels vary greatly, Garford Formation. with widths of the river valleys ranging from a few tens of meters When the Tertiary palaeochannels formed and for a time thereafter, to more than 30 km and depths of up to 120 m. In length, most the climate of southern Australia was warm and wet (e.g. Alley of the main channels extend for more than 100 km. These large et al., 1999; Frakes, 1999). The effects of this and later climatic sizes reflect past conditions of high precipitation as implied by regimes on weathering, erosion and sedimentation were of great the fossil evidence. The principal direction of drainage is towards significance. Key events in the history of the craton and the Eucla the southwest, but there is considerable variation, such as towards margin were episodic duricrust-forming episodes during the Early the northwest and south (Figure 1). The palaeochannel mouths and Late Eocene, the Middle Miocene and the Late Pliocene are characterised by a series of ancient estuaries (e.g. Wilkinson (e.g. Frakes, 1999). The evidence from Eocene megaflora and and Anthony) and coastal barriers (Ooldea and Barton ranges). microflora suggests at least local warm–temperate rainforest Between the coastal barriers lie NW–SE trending lagoons (Figure conditions (Alley et al., 1999); this is supported by the widespread 1). accumulation of lignites along the southern continental margin. Subsidence in the Eucla Basin probably commenced in the Cretaceous–Paleocene shortly before the earliest palaeovalleys

© CRC LEME 2003 Gawler Craton palaeochannels Page 3 formed, with on-going slight epeirogenic uplift of the cratonic inspiration derived from the study of palaeodrainage on margin (Frakes and White, 1997) and on-going incision of the the Gawler Craton, SA. In: H. Xie, Y. Wang and Y. Jiang valleys (Hou et al., 2001a). Prior to the Tortachilla marine (Editors). Computer applications in the minerals indus- transgression, deposition of fluvial coarse materials was limited tries. Proceedings of the 29th International Symposium on Computer Application in the Minerals Industries. A.A. to the valley thalweg. This implies that unless the fluvial deposits Balkema. pp 107-110. were originally thicker and subsequently eroded, there had been Hou, B., Frakes, L.A., Alley, N.F. and Clarke, J.D.A., 2003. no significant fluvial aggradation in the landward extremity of Characteristics and evolution of the Tertiary palaeovalleys the incised valley either during sea-level lowstand or during the in the NW Gawler Craton, South Australia. Australian subsequent Tortachilla sea-level rise (Hou et al., in press). Then, Journal of Earth Sciences, 50:215-230. the basin and channel sediments thicken and are inclined gently Hou, B., Alley, N.F., Frakes, L.A., Gammon, P.R. and Clarke, in a seaward direction during following Tuketja Transgression. J.D.A., in press. Facies and sequence stratigraphy of Erosion of the Eocene sediments occurred in the Middle Miocene Eocene palaeovalley fills in the eastern Eucla Basin, South and Early Pliocene when lacustrine sedimentation dominated in Australia. Sedimentary Geology. the lakes and channels. During the Late Miocene–Early Pliocene, Lintern, M.J. and Sheard, M.J., 1998. Silcrete – a potential the palaeochannels were fragmented into chains of relatively new exploration sample medium: a case study from the small lakes in which dolomite was deposited. Challenger gold deposit. MESA Journal, 11: 16-20. Rogers, P.A., 2000. Tertiary palaeodrainage of South Australia. Map sheet. 1:2 000 000 Series (2nd edition). Geological REFERENCES Survey of South Australia, Adelaide. Alley, N.F., Clarke, J.D.A., Macphail, M. and Truswell, E.M., 1999. Sedimentary infillings and development of major Tertiary palaeodrainage systems of south-central Australian. International Association of Sedimentologists. Special Publication 27. pp 337-366. Benbow, M.C., 1990. Ancient coastal dunes of the Eucla Basin, Australia. Geomorphology, 3:9-29. Benbow, M.C., 1993. Tallaringa, South Australia. Map sheet and explanatory notes. 1:250 000 geological series, Sheet SH53-5. Geological Survey of South Australia, Adelaide. Benbow, M.C., Lindsay, J.M. and Alley, N.F., 1995. Eucla Basin and palaeodrainage. In: J.F. Drexel and W.V. Preiss (Editors). The geology of South Australia, Vol. 2, The Phanerozoic. Geological Survey, South Australia. Bulletin 54. pp 178-168. Clarke, J.D.A. and Hou, B., 2000. Eocene coastal barrier evolution in the Eucla Basin. MESA Journal, 18: 36-41. Frakes, L.A., 1999. Evolution of Australian environments. In: A. Orchard (Editor). Flora of Australia. Australian Government Printing Office. pp 163-204. Frakes, L.A. and White, M., 1997. Tertiary palaeochannels draining the Gawler Craton. MESA Journal, 6: 10-11. Hou, B., Frakes, L.A., Alley, N.F., Stamoulis, V. and Rowett, A., 2000. Geoscientific signatures of Tertiary palaeochannels and their significance for mineral exploration in the Gawler Craton region. MESA Journal, 19: 36-39. Hou, B., Frakes, L.A. and Alley, N.F., 2001a. Development of geoscientific models for exploration in Tertiary palaeochannels draining the northwest Gawler Craton, S.A. Department of Primary Industries and Resources, South Australia. Report Book 2001/021. 133 pp. Hou, B., Frakes, L.A. and Dodds, A.R., 2001b. Recent drilling in the Garford Palaeochannel, Gawler Craton, – testing of a model derived from interpreted geological, geophysical and spectral methods. MESA Journal, 20: 24-27. Hou, B., Frakes, L.A., Alley, N.F. and Gray, D.J., 2001c. Development of integrated exploration methods: an